Surface chemistry of aqueous film coating suspensions

Archana Sharadchandra Desai, Purdue University

Abstract

As titanium dioxide (TiO$\sb2$) and aluminum lake dyes (lakes) impart color and opacity to films by being uniformly distributed and since they both exhibit pH-dependent surface properties, it is important to study their behavior in dispersions as related to surface charge. It was observed that the PZC of TiO$\sb2$ was lowered by surface anions. The PZC of the lakes was also inversely related to the anionic dye content. The effect of pH on bleeding of two aluminum lakes was investigated over a pH range of 2.0-8.0. The pH of minimum bleeding corresponded to the pH of minimum solubility of the aluminum-containing substrate. The similarity of the bleeding profile of both lakes indicates that pH related bleeding, other than that occurring by competition with anions, is a result of dissolution of the aluminum-containing substrate and is related to the substrate rather than to the structure of adsorbed dye. In suspensions containing TiO$\sb2$, an increase in bleeding was observed at pH conditions where TiO$\sb2$ was negatively charged. In suspensions where TiO$\sb2$ had a high positive charge, no difference in bleeding was observed in comparison to suspensions of only the lake at the same pH. The degree of bleeding was directly related to the magnitude of the negative zeta potential of TiO$\sb2$. It is hypothesized that negatively charged TiO$\sb2$ accelerates bleeding due to surface acidity. In these suspensions, the concentration of dye in solution increased with time but aluminum concentration decreased. The aluminum cations may be solubilized by the surface acidity of TiO$\sb2$ and adsorbed by the negatively charged TiO$\sb2$ particles. Evidence of this hypothesis is the change in zeta potential from negative to positive with time. The rheological behavior of these dispersions differed with the pH immediately after the dispersions were prepared indicating that aggregation depended on the pH. After aging there was no change in the rheology showing that particles were fully aggregated. The rate of aggregation depended upon the charge of TiO$\sb2$ and lake. Titanium dioxide and lake were incorporated in hydroxypropylmethyl cellulose films cast from dispersions adjusted to different pH values. Surface reflectance results illustrated differences between formulations.

Degree

Ph.D.

Advisors

Hem, Purdue University.

Subject Area

Pharmaceuticals

COinS